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5 Geodetic Reference Frames and Co-Location Requirements
Pages 89-100

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From page 89...
... The ITRF is created and maintained, or "real ized," by using geodetic observations to determine the positions and velocities of physical reference points on the Earth's surface, and matching them as closely as possible to the mathematical and physi cal properties of an idealized, or theoretical, frame. The reference points for the ITRF may consist of geodetic equipment on the ground, or fixed points within the geodetic instruments themselves.
From page 90...
... Given that existing reference frames have not achieved this level of accuracy and stability, it is not surprising that one of the largest sources of error in the global characterization of longterm sea level variation is uncertainty in the ITRF. For example, a 2-millimeter-per-year error in the relative velocity between the Earth's mean surface and the Earth system's center of mass can result in an error as large as 0.4 millimeters per year in the determination of mean global sea level variation using satellite altimetry (see Table 3.1)
From page 91...
... GEODETIC TECHNIquES FOR REALIZING THE ITRF The geodetic techniques that provide measurements for realizing the ITRF are Very Long Baseline Interferometry (VLBI) ; Global Navigation Satellite Systems (GNSS)
From page 92...
... For example, any scale bias in the ITRF definition propagates directly to the height component of the stations, and any scale and/or origin bias will directly map to the mean sea level estimation (Beckley et al., 2007)
From page 93...
... . The national agencies that operate geodetic instruments generally perform least-squares adjustments of local surveys to yield the local ties that connect co-located instrument reference points.
From page 94...
... 1ITRF Product Center: http://itrf.ensg.ign.fr/. 2Based on the difference between local tie measurements and geodesy estimates, assessed via the Weighted Root Mean Scatter of the tie residuals as results from the ITRF combination (Altamimi et al., 2002, 2007)
From page 95...
... GRASP proposes to co-locate VLBI, GNSS/GPS, SLR, and DORIS sensors on a well-calibrated spacecraft (for which internal offsets are measured very accurately) , to establish precise, stable ties between the key geodetic techniques used to define and disseminate the ITRF.
From page 96...
... ; at least three GNSS/GPS receivers to provide local tie information and monitor site deformation; a DORIS beacon; terrestrial survey instruments to determine and monitor local ties to the millimeter level; a superconducting or, preferably, an absolute gravimeter; meteorological sensors; and a variety of other sensors, such as seismometers, tiltmeters, and water vapor radiometers (see also Plag and Pearlman, 2009)
From page 97...
... reflectors could help specifically to improve the GPS satellite transmitter phase center modeling and to refine orbit modeling. REGIONAL REFERENCE FRAMES AND THEIR RELATIONSHIP TO THE ITRF Regional and national geodetic reference systems are essential for a variety of civil, legal, and public safety applications.
From page 98...
... Twenty-five years later, it would be timely and appropriate to upgrade the NAD system again, taking advantage of the latest global and national geodetic observations and geophysical models. The NAD83 differs from the ITRF at the level of one meter, a very large number considering the approximately one-millimeter level of precision of today's geodetic techniques.
From page 99...
... You can't see it, touch it or even dig down to find it. Simply put, the geoid is the natural extension of the mean sea level surface under the landmass.
From page 100...
... . In effect, GRAV-D would provide the data necessary to define the national height datum as the new NAD ellipsoidal surface plus a geoid correction, modeled using gravimet ric measurements.


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